Cyclical obstruction communication system

ABSTRACT

Techniques for improving data rates at mobile terminals that are subject to periodic channel interruptions in a beyond-line-of-sight communication system are disclosed, including improved encoding and decoding systems that identify blockages and modify receiver operation during blockages to reduce data errors. In certain embodiments, encoding, symbol mapping, interleaving, and use of unique periodic identifiers function to enable a series of packets that may be received in a blockage impaired channel with reduced errors.

CROSS REFERENCES

This application claims the benefit of and is a continuation of U.S.patent application Ser. No. 13/540,541 filed on Jul. 2, 2012, which is acontinuation-in-part (CIP) of U.S. patent application Ser. No.12/446,170 filed on Apr. 17, 2009, which is a U.S. National PhaseApplication of PCT/US2007/080720, filed Oct. 22, 2007, which in turnclaims priority of U.S. Provisional Application Ser. No. 60/862,452filed on Oct. 22, 2006, all of which are assigned to the assigner hereofand expressly incorporated herein by reference for all purposes.

BACKGROUND

The present invention relates generally to satellite communications and,more specifically, to a beyond line of sight (BLOS) communication systemin which mobile terminals can experience channel blockage events.

Developments in broadband beyond line of sight communications havefocused mainly on ground-mobile platforms and, to a lesser extent, onmaritime vessels and fixed wing aircraft. For example, in recent years,there have been advances in precision tracking antenna technology. Someprogress has also been made in the area of waveform construction andprotocol performance for the ground-mobile environment.

Advances relating to rotary-wing aircraft and other platforms thatexperience occlusions have been conspicuously absent. Helicoptercommunications, for example, can experience periodic outages such aswhen the rotor blades block the antenna, or when the orientation of theaircraft changes relative to a communications satellite. Railcommunications may experience interruptions due to trellises or otheroverhead structures.

As a result, these platforms have been relegated to relatively low datarate systems such as UHF SATCOM (Ultra High Frequency SatelliteCommunications) and Inmarsat's L-band GAN (Global Access Network)service. Conventional systems such as these can only offer data rates ofbetween 64-384 kbps and have thus failed to keep pace with the rapidlyincreasing demand for bandwidth required to support modem userapplications.

SUMMARY

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the following drawings. In theappended figures, similar components or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

FIG. 1 is a high-level block diagram depicting a satellite communicationsystem according to one embodiment of the present invention.

FIG. 2 is a diagram illustrating periodic blockage of a communicationchannel such as may be experienced by a rotary-wing aircraft.

FIG. 3 is a block diagram of a data packet destined for an obstructedterminal according to one embodiment of the present invention.

FIG. 4 is a flowchart showing forward link transmit processing accordingto one embodiment of the present invention.

FIG. 5 is a flowchart showing forward link receive processing accordingto an embodiment of the present invention.

FIG. 6 is a block diagram of a transport processor according to oneembodiment of the present invention.

FIG. 7 is a block diagram of a satellite communication system accordingto one embodiment of the present invention.

FIG. 8 is a diagram illustrating return link processing in accordancewith embodiments of the present invention.

FIG. 9 is a block diagram of a system for communication over a blockageimpaired channel in accordance with embodiments of the presentinvention.

FIG. 10 is a block diagram of an interleaver for communication over ablockage impaired channel in accordance with embodiments of the presentinvention.

FIG. 11 is a block diagram of a deinterleaver for communication over ablockage impaired channel in accordance with embodiments of the presentinvention.

In the figures, similar components and/or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label by a dash and a secondlabel. If only the first reference label is used in the specification,the description is applicable to any one of the similar componentshaving the same first reference label.

DETAILED DESCRIPTION OF THE INVENTION

Satellite communications with rotary-wing aircraft such as helicopterscan be benign, difficult, or very difficult depending on what operatingfrequency is contemplated. This is due to the relative electrical sizeof the rotor blade compared to the wavelength of the signal being usedto transmit and receive to/from the helicopter. For example, at UHF, thewavelengths are long electrically compared to the rotor blade and thussignificant diffraction of the signal occurs around the blade. As aresult, only a very small degradation is experienced when the UHFantenna is placed beneath the rotor blades. It is therefore possible touse UHF systems on helicopters without significant, if any, modificationto the waveform.

L-band is a higher operating frequency and has a correspondingly shorterwavelength. The rotor blade thus appears electrically larger, resultingin less diffraction and more of a “fade” when the blade passes over theantenna. This fade is typically on the order of several dB. Thus, it isstill possible to operate an L-band system through rotor blades ifenough margin exists in the link budget to “burn through” the fade.However, like UHF systems, L-band communications manage to achieve onlylimited data rates.

Fade depth grows with frequency. Beyond L-band, there becomes a pointwhere sufficient link margin cannot be maintained (or it is tooexpensive or difficult) to simply burn through the fade. At Ku-band, forexample, the fade caused by rotor blades has been measured to exceed 10dB. Margin is not generally built into the link budget to accommodatethis level of fade.

The structure of the rotor itself also has a tremendous effect on theamount of diffraction and thus the depth of the fade. However, rotorblades are designed primarily for flight safety and it is generally notpossible to modify them for the purpose of improving communications.Therefore, under normal conditions, a satellite antenna mounted in thevicinity of rotor blades will experience significant blockages as theblades pass over it.

Adding to the problem, antennas sized large enough to achieve truebroadband data rates, yet small enough to be compatible with helicopteroperation, are not generally available below Ku-band frequencies.

Depending on the orientation of the aircraft relative to the satellite,the outage encountered by the satellite terminal may varypercentage-wise from a few percent to twenty-five percent or more (ratioof blockage time to blockage period) at low elevation angles or extremeaircraft orientations. Outages of more than twenty-five percent can beconsidered effects similar to those encountered in the ground-mobileenvironment such as driving by trees and buildings, below overpasses, orparking behind obstructions. These extreme cases are likely to beencountered only for limited periods of time.

In addition, two-rotor aircraft will generally exhibit a lower blockagepercentage than four (or more) rotor aircraft. While the blockage periodis often consistent for a given aircraft, it can vary from aircraft toaircraft of the same make/model and can vary significantly betweenaircraft models. Some rotary-wing aircraft also exhibit varyingperiodicity of the blockage when maneuvering by upspeeding/downspeedingthe rotor. Accordingly, as a general constraint, a Ku-band (or similarhigh frequency) communication system should accommodate periodic outageson the order of several percent to at least twenty-five percent.

FIG. 1 is a high-level block diagram depicting a satellite communicationsystem 100 according to one embodiment of the present invention. Asshown, satellite communication system 100 uses a hub-spoke topology inwhich a hub/gateway 110 communicates with a plurality of mobileterminals 130, 140, 150 via satellite 120. For convenience, thecommunication paths from gateway 110 to mobile terminals 130, 140, 150will be referred to as forward links. Similarly, the communicationspaths from mobile terminals 130, 140, 150 to gateway 110 are describedas return links. Accordingly, communication between gateway 110 and aparticular mobile terminal 130, 140, 150 can be characterized in termsof its forward link, return link, or with reference to both directions.

Gateway 110 receives data packets for transmission to mobile terminals130, 140, 150. As used herein, the term “packet” or “data packet”describes a unit of addressable data which can be sent or receivedwithin satellite communication system 100. Packets may be defined at thetransport layer, at the network layer, or in accordance with aparticular channel access protocol. For example, packets may include IPpackets, HDLC (high-level data link control) frames, ATM (asynchronoustransfer mode) cells, or units of data characterized by time and/orfrequency slots such as in a multiple access system. Packets can alsoinclude sub-units. In particular, packets may describe sub-unitsproduced by data fragmentation techniques. Packet addressing(“packetization”) may be performed at the data source or, in someembodiments, by one or more processing layers of gateway 110.

As depicted in FIG. 1, gateway 110 is coupled to and receives packetsfrom external network 190. Gateway 110 processes the packet data andsends it to its destination within communication system 100 viasatellite 120. In the return direction, data from mobile terminals 130,140, 150 is received at gateway 110 for delivery over network 190. Amongother elements, gateway 110 includes hardware and software to detect andreceive return link signals, provide forward link transmission, andmanage network traffic. Network 190 can be a public network such as theInternet, or it may provide dedicated circuit backhauling into a privatenetwork.

Mobile terminals 130, 140, 150 are shown as including airborne as wellas ground-based communication platforms. Mobile terminal 150 isvehicle-mounted such as can be used with a truck or rail transport.Mobile terminals 130, 140 are adapted for use with aircraft. Forexample, mobile terminal 140 can be used with a fixed-wing aircraft suchas an airplane while mobile terminals 130 a, 130 b can be fitted tohelicopters or other rotary-wing aircraft. Communication system 100 istherefore a heterogeneous system in that it is adapted for use withremote terminals which may be subject to periodic outages or otherservice interruptions as well as terminals that typically do notexperience such outages. Although not shown, embodiments of satellitecommunication system 100 can include other communication platforms suchas ship-based (maritime) terminals in addition to land-based terminals.

In some embodiments, mobile terminals 130, 140, 150 are classified bygateway 110 according to whether they are subject to periodic outages.Thus, for example, helicopters 130 may be classified as “obstructed”terminals due to the periodic blockages caused by their rotor blades.Airplane 140, on the other hand, may be classified as “non-obstructed”based upon its flight characteristics and the placement of its antenna.Vehicle 150 may be classified as either obstructed or non-obstructedaccording to circumstances. For example, a train with frequent blockagessuch as those caused by periodically spaced overhead electrical powertrellises may be classified as “obstructed” whereas a truck operating inrelatively open areas may be classified as “non-obstructed.” The mannerin which these classifications are used is discussed below.

Mobile platforms can include a satellite modem, a power distributionsystem, and baseband equipment such as TCP accelerators, routers, andencryption devices. Advantageously, from the user's standpoint, mobileterminals 130, 140, 150 may provide standard, cable modem-type networkaccess and transparently support commercial and military softwareapplications at true broadband data rates. With satellite communicationsystem 100, obstructed terminals such as helicopters 130 communicate atbroadband data rates without reducing the performance of non-obstructedterminals such as fixed-wing aircraft 140. In an exemplary embodiment,each mobile terminal includes a satellite modem incorporating ArcLight®technology from the ViaSat corporation of Carlsbad, Calif.

Satellite communication system 100 may utilize different waveforms forthe forward and return links. In the presently described embodiment,gateway 110 generates a single, high-rate time-division multiplexed(TDM) signal 160 a that is relayed by satellite 120 to all the servicedusers in the forward direction. Mobile terminals 130, 140, 150 receiveand demodulate the high-rate forward signal 160 b to recover packet dataaddressed to them. Forward link 160 may employ a variety of modulationand pulse shaping techniques, including spread spectrum techniques. Inan exemplary embodiment, forward link 160 is a Ku-band signal andsupports information rates of up to 10 Mbps. Other forward link transmitwaveforms are possible and may include, for example, channel accessprotocols such as TDMA (time-division multiple access) and MF-TDMA(multi-frequency time-division multiple access). With an MF-TDMAapproach, gateway 110 may serve as a bandwidth manager and packets maybe “addressed” based upon the particular time and frequency slot theyoccupy.

In an exemplary embodiment, mobile terminals 130, 140, 150 each transmita CRMA (code-reuse multiple access) signal to satellite 120 on theirrespective return links 170. Satellite 120 relays a composite signal 180(which may be the sum of all the CRMA signals 170 from the mobileterminals) back to gateway 110. With CRMA, each mobile terminal 130,140, 150 decides locally when to transmit without requiring bandwidthassignment from gateway 110 or satellite 120. Thus, in some embodiments,the return link signals are locally-controlled asynchronous burstsgenerated by mobile terminals. Alternatively, TDM, TDMA, and MF-TDMAsignals may be used on the return link.

Although depicted as having a hub-spoke architecture, persons of skillin the art will recognize that communication system 100 supports otherarchitectures and network topologies. Also, while Ku-band transmissionis believed to represent a good balance of data rates and antenna sizeswhile still achieving regulatory compliance, other operating frequenciesare specifically contemplated for use with embodiments of the presentinvention. For example, among others, embodiments of the presentinvention may use commercial C-band, X-band, and Ka-band frequencies toachieve high data-rate communications in the presence of blockages.

A continuously transmitting waveform (such as forward link 160) may besubject to periodic blockages. For example, blockages can result whenhelicopter blades obstruct a satellite antenna. These periodic blockagesresult in large numbers of dropped packets and can also cause linktracking failures. Also, if not properly managed, periodic blockages canresult in a catastrophic link loss such that a receiver is unable toreacquire the forward link signal even after the blockage has passed.

Unlike conventional approaches, embodiments of the present inventionaddress these problems directly using a combination of inventivetransmit and receive techniques. For clarity, the following discussionis provided from the standpoint of time-diversity implemented on theforward link. However, it will be recognized that time-diversitytechniques may be advantageously used for the return link as well. Forexample, various embodiments of the present invention use time-diversitytechniques on the return link at obstructed mobile terminals.

Time diversity in one embodiment involves transmitting a data packet ata first time and then transmitting the same data at a carefully selectedsecond time. The second time can be determined based upon blockagecharacteristics detected either at the gateway or at the mobileterminal. For example, in one approach, a hub (such as gateway 110)monitors burst arrivals, burst timing, packet counts, and feedbacksignals from an obstructed terminal and forms an estimate of theblockage duration on the basis of such communication activity. The hubmay track each terminal's communications (or communication patterns)over time and dynamically update the blockage parameters when changesare detected. In this way, the blockage estimate used to determine thesecond transmit time can be successively updated and improved.

Alternatively, blockage parameters can be measured at the mobileterminal and reported to the hub/gateway for use in timing thetransmission of duplicate packets. For example, when an obstructedmobile terminal such as a helicopter establishes a connection with thehub, either during a login or as part of a handshake procedure, a delayvalue for time-diversity transmissions can be established. Theobstructed terminal can provide information about the duration of itsblockage period, the duration of the blockage event, and/or the durationof its open interval for use by the hub in determining the appropriatedelay based upon prevailing conditions.

When a continuously transmitting forward link is utilized, theobstructed terminal (e.g., helicopter 130) can measure its blockageparameters based upon detecting the presence of the forward link signaland report them back to the hub/gateway (e.g., gateway 110). Otherapproaches to gathering data about the obstruction at the mobileterminal can include mechanical sensors, optical sensors, or othernon-communications based techniques which measure the physical locationof the helicopter blade relative to the satellite antenna. Theobstructed terminal can send blockage data and additional relevantinformation such as flight characteristics of the aircraft, rotationalspeed of the rotor, number of rotor blades, etc. Based upon informationfrom the mobile terminal, the gateway can determine the appropriatetime-diversity delay for avoiding the periodic obstruction. Whetherperformed at the hub or at the mobile terminal, information gatheringand reporting activities may be ongoing so that the blockage estimateremains up-to-date and reflects current operating conditions.

As provided in embodiments of the present invention, time-diversity isnot a “blind” approach based simply upon repetitive data transmission.By contrast, embodiments of the present invention use intelligenttime-diversity in which data transmission is carefully controlledaccording to blockage characteristics, prevailing conditions at themobile terminal, and other factors bearing upon the mobile terminal'sability to receive a communication signal. Unlike blind time-diversity,the inventive approach reduces the effect of the obstruction whileconserving scarce bandwidth resources.

In addition, embodiments of the present invention may utilizetime-diversity without changing the physical layer waveform. Instead, anexisting waveform can be used to carry one or more copies of thetime-diversity data. A data-centric approach to time-diversity canadvantageously be implemented without extensive equipment modificationand, as discussed below, it can be used in a selective manner. Personsof skill in the art will recognize that physical-layer changes affectall communication terminals in the same manner and therefore can beexpensive and inflexible.

Based upon empirical data, the inventors of the present invention havedetermined that blockages such as those experienced at helicopter-basedterminals under normal operating conditions may vary up to abouttwenty-five percent. Extreme blockages can be considered to be effectssimilar to those encountered in the ground-mobile environment and arelikely to prevail only for limited periods of time. Using time diversitytechniques, however, embodiments of the present invention can supporthigh data rate transmission into obstructed terminals with blockageratios approaching 50% of the blockage period. Thus, time-diversity canbe used to avoid a large number of blockage (or signal degradation)events.

FIG. 2 is a diagram illustrating blockage of a communication channelsuch as can be caused by the blades of a helicopter. As shown, timepasses from left to right. At initial time t(n), the nth helicopterblade pass has just begun and the blade obstructs the helicopterantenna. The channel thus enters the blocked state at this time. Afterthe blade has passed beyond the antenna, the channel enters the open(unblocked) state allowing communication signals to be transmitted andreceived. When the (n+1)th helicopter blade pass begins at time t(n+1),the channel again enters the blocked state. The time between the startof the initial and subsequent blade pass events is termed the blockageperiod and is labeled as T.sub.block.sub.- -.sub.period. Similarly, theblockage duration is shown as T.sub.blocked, and the open (unblocked)interval is denoted T.sub.open.

Given the short-term stability of rotor passes and the relatively shortblockage duration relative to the blockage period, a range oftime-diversity delay values (T.sub.delay) can be adopted. An exemplaryrange of time-diversity delay values is provided by the followingexpressions in which delay is measured from the time at whichtransmission of the original packet begins. These bounds can be readilyappreciated with reference to FIG. 2.

1. T.sub.delay>T.sub.packet+T.sub.blocked+.DELTA.

2. T.sub.delay<T.sub.block.sub.--.sub.period-T.sub.packet-T.sub.block-ed-.DELTA.

where: T.sub.packet represents the time required to transmit a datapacket at a specified data rate T.sub.block.sub.- -.sub.periodrepresents the period of the blockage event T.sub.blocked represents theduration of the channel blockage .DELTA. represents the demodulatorrecovery time.

Time-diversity delay values can be set within the range defined byExpressions 1, 2. The first expression represents a minimum delay afterwhich a duplicate packet should be transmitted to ensure that either theoriginal packet or the duplicate packet will be received in the presenceof the periodic blockage. Similarly, the second expression is an upperbound and establishes a maximum delay that will result in either theoriginal or duplicate packet avoiding the periodic blockage with aminimum of delay, i.e. incorporating at most single blockage event. Inaccordance with the foregoing, it will be understood that the delayvalue can be increased by an integral number of blockage periods(T.sub.delay=T.sub.delay+n*T.sub.block.sub.- -.sub.period) withoutdeviating from the principles described herein.

The time required to transmit a packet of a given size (T.sub.packet)varies according to the data rate of the communication system. In somecases, packet transmission time T.sub.packet can be conveniently setaccording to a maximum packet size parameter of the communicationsystem. In addition, the demodulator recovery time (.DELTA.) isgenerally known, but can be set to a reasonable value determinedaccording to the expected data rate and other relevant factors.

As an alternative to Expression 2, a delay value based upon a 50%blockage ratio can be used to establish the upper-bound of the delayvalue.

3. T.sub.delay<T.sub.block.sub.- -.sub.period/2

With this approach, T.sub.delay is set within the range defined byExpressions 1, 3. Expression 3, for example, may be useful in cases whenthe blockage period is known but other blockage parameters, such as theblockage duration, are not known. Alternatively, Expression 3 may beused an initial delay as more precise blockage data is acquired.

Following this approach, in an exemplary embodiment of the presentinvention, an obstructed terminal communicates the period of itsblockage (T.sub.block.sub.- -.sub.period) to the gateway when acommunication session is established. The gateway establishes theinitial time diversity delay value for the obstructed terminal withinthe range defined by Expressions 1, 3 above. Thereafter, the gateway mayacquire new blockage data either from the obstructed terminal directly,or by monitoring the communication activity of the obstructed terminaland generating estimates of its relevant blockage characteristics. Usingthe new blockage data, the gateway may update the time-diversity delayused with the obstructed terminal. For example, as more blockage data isacquired, the new time-diversity delay value may be established usingthe range defined by Expressions 1, 2.

The time-diversity delay value can be used, for example, to set thedepth of a delay buffer or the value of a programmable timer such that aduplicate packet is transmitted after its original packet by the delayvalue T.sub.delay. This ensures that either the original or duplicatetime-diversity packet avoids the periodic obstruction and arrives at itsdestination By ensuring that at least one time-diversity packet isreceived, sustained high data rate communications are possiblenotwithstanding the effect of the periodic obstruction. One exemplaryembodiment of the present invention operates in the Ku-band and canachieve information rates of up to 10 Mbps to unobstructed terminals,and time-diversity rates of up to 5 Mbps into obstructed terminals.

Time-diversity can be implemented at the network layer or at thetransport layer and can be performed in a brute-force or a selectivemanner. For example, if IP networking is used, time-diversity processingcan be performed at the IP layer. Transport layer processing, amongother possibilities, can include the marking and manipulation of HDLCframes. In addition, time-diversity can be applied to all or only somedata packets. A brute-force approach transmits duplicate packetsregardless of their destination. In other words, no distinction is madebetween obstructed and non-obstructed terminals. On the other hand,time-diversity can be advantageously applied such that only packets sentto/from obstructed terminals are duplicated and no duplicate processingis needed at the non-obstructed terminals.

Selective time-diversity can be implemented in heterogeneous networks atthe IP layer in several ways. One approach is to classify incoming IPpackets according to their destination. For example, a hub (such asgateway 110) can monitor the IP destination address field of eachincoming packet. If the destination address corresponds to a terminalknown to be obstructed, such as a helicopter, the incoming packet can bestored in an obstructed-group buffer. Alternatively, if the incomingpacket is not destined for an obstructed terminal, it can be added to anon-obstructed buffer.

Next, a payload can be created by allocating a fixed percentage of eachtransmit burst for obstructed packets and filling the remainder withnon-obstructed packets. The quantity of obstructed packets can belimited according to the percentage allocation. For example, 20% of thepayload can be reserved for obstructed packets. In cases where theobstructed portion of the payload has been filled and there areinsufficient non-obstructed packets to fill the remainder, additionalobstructed packets can be added. Of course, if the portion allocated forobstructed packets is not filled, non-obstructed packets may be added.The percentage of the payload allocated to the two packet types can be afixed value or dynamic based on IP traffic composition and/or effectiveover the air rate matching. Provision can also be made to provide aminimum percentage allocation to the two packet classifications to avoidstarvation. In creating the payload, high priority packets (such ascommand, status/control, or those destined for priority-users) can beadded before low priority packets. Quality of service markings, such asthe IP DiffServ field, can also be utilized when assembling the payloadto facilitate bandwidth management.

Some embodiments of the present invention transmit time diversity datain blocks. Each block typically comprises a group of IP packets destinedfor obstructed receivers in a first burst, followed by the same group ofIP packets (duplicate packets) which are delayed and sent in a secondburst. The delay can be determined based upon the periodic obstructionusing the techniques previously disclosed and ensures that either theoriginal or the duplicate packet is received at the obstructeddestination. With selective time-diversity, packets destined forobstructed terminals are transmitted twice per block, whereas each burstin a block includes new data for non-obstructed terminals. In this way,the communication system can support a mixture of obstructed andnon-obstructed terminals without a corresponding reduction in data ratefor non-obstructed terminals.

Various approaches can be used to control the timing at which each burstis transmitted. For example, a counter can be used such that a burst ofpackets is transmitted at a predetermined bit count. Alternatively, aprogrammable timer can be used to interrupt a transmit processor andcause it to send a burst of packets. When using a timer, the duration ofa burst cannot exceed the period of the timer. Thus, with this approach,the quantity of packet data (and therefore the size of the payload) thatcan be transmitted in a burst is determined by the timer period and theover-the-air data rate.

FIG. 3 is a block diagram of a data packet 300 destined for anobstructed terminal according to one embodiment of the presentinvention. As shown, control information can be added to obstructedpackets as part of time diversity processing to assist in packetreconstruction at the receiving terminal. Header 310 and payload 320represent elements of a standard IP data packet. Control structure 330can be appended to the data packet or encapsulated within the packet andcan contain a variety of data fields useful for performing packetreconstruction. For example, in one embodiment, control structure 330includes an obstructed/non-obstructed field, an original/duplicatefield, a sequence identifier, a burst identifier, and one or morefragmentation flags. Thus, during time diversity processing, eachobstructed packet is marked as either an original or a duplicate packetand is assigned appropriate identifiers.

FIG. 4 is a flowchart showing transmit-side processing 400 oftime-diversity packets according to embodiments of the presentinvention. The processing steps shown represent a brute-force(non-selective) approach which can be implemented at either a gateway ora mobile terminal. Referring again to FIG. 1, it will be noted thattime-diversity transmission can be used on the forward link, the returnlink, or both in satellite communication system 100 according toembodiments of the present invention. It will also be noted thattime-diversity may be advantageously used with multiple accesstechniques such as TDMA or MF-TDMA. In an exemplary communicationsystem, gateway 110 is configured for selective time-diversitytransmission, obstructed terminals 130 are configured for brute forcetime-diversity transmission, and non-obstructed terminals 140, 150 donot utilize time-diversity on either the forward or return links.

In a first step 410, a packet is received for over-the-air transmissionfrom the gateway or hub. The packet may be received from an externalnetwork such as the Internet for delivery to an obstructed mobileterminal such as helicopter 130. At step 420, a packet delay value isdetermined based upon blockage characteristics. As previously discussed,the delay value may be supplied to the gateway by a mobile terminal orinferred based upon a mobile terminal's communication patterns.

After being received, at step 430, the packet is placed into a transmitbuffer. The transmit buffer stores a first copy (“original”) of the datapacket and may be organized in a variety of different ways to observepacket-priority or quality of service constraints within thecommunication system. In addition, a second copy of the packet(“duplicate”) is added to a delay buffer 440 for transmission after theoriginal packet. The delay buffer implements the transmit delaydetermined at step 420 to provide the appropriate time spacing betweenthe original and duplicate packets based upon blockage characteristics.

Prior to transmission, a control structure is added to both the originaland duplicate packets 450 to facilitate reconstruction by the receiver.The control structure includes an indication of whether the packet is anoriginal or duplicate copy, and provides identifiers used for packetreconstruction. Identifiers, for example, may include a sequenceidentifier which provides a temporal sense of the packet relative toother packets in a burst transmission as well as a maximum count of thenumber of packets included. In some embodiments, a packet's positionrelative to other packets in the burst can be determined according thesequence identifier and its “membership” in a group of packets can bedetermined by its burst identifier. The receiver can use thisinformation to identify a packet as the “next” packet in the receivesequence or as the “last” packet in a particular group. The receiver canalso detect the arrival of a new group of packets using the identifiers.Packets with the same burst and sequence identifiers, for example,contain the same data.

The original packet is transmitted at a first time, step 460, before theduplicate packet. At step 470, the transmit delay is implemented.Thereafter, at step 480, the duplicate packet is transmitted at a secondtime following the delay. In this way, at least one of the original orduplicate packets will avoid the obstruction. Although these steps410-480 generally describe the technique applied to the forward linkportion of the link (160), they can also be applied to the return links170 with minor changes. In particular, steps 410-480 may beadvantageously used with time-division multiple access techniques aspreviously discussed.

Time-diversity at the IP level has limits with respect to the over theair data rate. Generally, the transmit time for the largest IP packetmust fit within the limits set by the obstruction periodicity. For lowOTA data rates and large packet sizes, time-diversity can be adjusted toaccommodate these constraints. One approach involves IP fragmentation.In effect, large IP packets are broken up into a collection of fragmentsat the IP-stack level using standard IP fragmentation techniques.Alternatively, a second approach performs fragmentation after packetshave left the IP stack and are ready to be transmitted. This avoids theproblems of IP fragmentation where an active ‘Do Not Fragment’ IPcontrol flag would prevent the IP stack from creating smaller packets.Post-IP stack fragmentation at the transmitter requires packet fragmentreconstruction at the receiver before packets are passed to thereceiver's IP stack. In both cases, fragmentation is configured todivide IP packets into smaller pieces (fragments) according to theover-the-air data rate and the “open” or unobstructed portion of theblockage period so that data transfer is optimized.

FIG. 5 is a flowchart showing receive-side processing 500 oftime-diversity packets according to an embodiment of the presentinvention. Time diversity techniques present challenges for thereceiver. One challenge is the possibility that more than one of thesame packet may be received. In other words, both the original and theduplicate time diversity packet could be received. Thus, the receivermay have an ability to judge if two packets are related (as original andduplicate) and to track whether one or possibly both packets have beenreceived. Additionally, the receiver may need to deal with packets whicharrive out-of-order. The receive-side processing 500 has both of thesefeatures.

Processing begins at step 510 when a first time-diversity packet isreceived. This packet may be followed by other original/duplicatepackets. At step 520, the receiver waits for the arrival of a duplicatepacket corresponding to the first original/duplicate packet. Forexample, if the first packet is marked as an original packet, thereceiver waits for the corresponding duplicate packet to arrive. If thefirst packet is marked as a duplicate, the receiver may not wait foradditional related packets to arrive. In one embodiment, original andduplicate pairs are identified based upon packet control informationsuch as a sequence identifier. Thus, for example, arriving packets maybe stored in a buffer at a position determined by their respectivesequence identifiers. The receiver then determines whether one copy ofall packets having the same identifier have been received with referenceto the buffer.

When one complete set of data packets is received, step 530, packet datacan be released for further processing. For example, after receiving thelast packet, the entire group of packets may be released for processing.However, the receiver does not wait indefinitely. If all the packetswith the same identifier are not received within a maximum period oftime then, at step 530, the packet data is released for processing. Insome embodiments, the maximum wait is set according to the depth of adelay buffer. For example, buffer depth may be established when thecommunication session begins and then used to measure how long it shouldtake for both packets to arrive. Alternatively, if packets in a new setare received before the old set is complete, packets in the old set maybe either discarded or released for processing.

At step 550, data is released to the system in the correct temporalorder. Following one approach, packet data is grouped according to aburst identifier included as part of the packet control information.Individual packets are assembled within the burst according to theirrespective sequence identifiers. When the entire burst has been receivedand assembled in the correct order, data can be released to the IPstack. Release of packet data can be metered to avoid IP stackcongestion or overflow, and a timer can be configured to flush packetsout of the buffer in the event of a defective block transfer.

Of course, receive-side processing can be extended to include IPfragmentation. In that case, as an additional processing task, thefragments are first assembled into packets. If standard IP fragmentationwas used at the transmitter, then the fragments may be reassembled atthe IP-stack level. Alternatively, if post-stack fragmentation wasperformed, packets are reassembled from the received fragments beforedelivery to the IP-stack. Control information such as fragmentationflags can aid in this process as known in the relevant art.

Accumulating an entire burst of data before releasing it to the systemminimizes jitter. However, it does not minimize delay. Therefore, insome embodiments, packets are released to the system as soon as they areneeded. With this approach, the “next” packet in a data stream isidentified according to its sequence and burst identifiers. When apacket arrives, if it is the next packet, it is immediately released forprocessing. If the packet is not the next packet, it is stored in areceive buffer. Packets in the receive buffer are then read out of thebuffer as they become the next packet. Subsequently received duplicatecopies are discarded. In this way, packet data is presented to thesystem as soon as it becomes available and in the correct order.However, the tradeoff can be higher levels of jitter.

As an alternative to IP-layer processing, time-diversity can beimplemented at the transport layer. Data frames, such as HDLC(high-level data link control) frames, are duplicated, delayed, andtransmitted in generally the same manner as described for IP packetswith appropriate hardware modifications for transport layer operations.As with IP-layer processing, time-diversity at the transport layer canbe implemented in a selective or brute-force manner and uses blockagecharacteristics to overcome obstructions. Transport layer data framesare recovered and placed in the proper order before being released tothe system as previously discussed.

FIG. 6 is a block diagram of a transport processor 600 such as can beused with embodiments of the present invention, including as part ofgateway 110. As shown, IP data packets arrive asynchronously at a ratebuffer 610 which provides an input to transport processor 600. At theoutput, packet and null data is transmitted in a synchronous stream offramed packets such as continuously transmitting forward link 160.

Rate buffer 610 provides the packets to framer 620 which assemblestransport layer frames and adds appropriate control information. Forexample, framer 620 accepts IP packets and generates HDLC frames at itsoutput. Time diversity HDLC frames are “marked” for duplication byadding appropriate control information. One output of framer 620 iscoupled to synchronous control element 650 and asserts a signal whenHDLC frames are ready to transmit. A second output of framer 620 iscoupled with multiplexer 670. The output of synchronous control 650forms the select input of multiplexer 670 such that available HDLCframes are output; otherwise, in their absence, null data is output. Thenull data is generated by element 660 and supplied at the second inputof multiplexer 670.

Filter 630 is coupled to the output of framer 620 and is configured tocopy marked data frames to delay buffer 640. In addition, filter 630marks the data frames it copies to delay buffer 640 as duplicates by,for example, setting appropriate control information bits. In this way,delay buffer 640 receives and stores data frames output by framer 620and which are marked both as duplicates and as obstructed.

The output of delay buffer 640 is coupled to a first input ofmultiplexer 690 and the second input of multiplexer 690 is coupled withthe output of multiplexer 670. The output of multiplexer 690 forms theoutput of transport processor 600. Thus, in operation, multiplexer 690supplies a synchronous stream including packet data (original orduplicate) and, when packet data is not available, null data.

Filter 630 also provides input to duplicate scheduler 680 which, inturn, controls the select input of multiplexer 690. When filter 630processes a marked frame, it notifies duplicate scheduler 680. Inresponse, duplicate scheduler 680 computes a future transport bit countat which the duplicate data frame will emerge from delay buffer 640. Thetransport bit count represents a time-diversity delay which will ensurethat either the original or the duplicate data frame are received in thepresence of the obstruction and is determined using one of theapproaches previously discussed.

When the bit-counter reaches the computed value, duplicate scheduler 680performs two actions. First, it temporarily disables the rate buffer 610and framer 620 interface so that new IP datagrams are not received intothe transport processor. Second, it causes multiplexer 690 to output theduplicate data frame by selecting at multiplexer 690 the input that iscoupled to delay buffer 640. After the duplicate copy has been insertedinto the transport stream, the rate buffer/framer interface is enabledand normal operation resumes. In this way, time-diversity processing atthe transport layer is realized in the communication system.

FIG. 7 is a block diagram of a satellite communication system 700according to one embodiment of the present invention. IP packetsreceived at hub 710 are transmitted on the forward link to communicationsatellite 750 and from communication satellite 750 to mobile terminal760.

Transport processor 720 receives incoming packets and prepares them fortransmission. This may include, for example, framing the data at eitherthe network or transport layers and scheduling burst transmissions asdescribed in connection with transport processor 600. Forward ErrorCorrection (FEC) encoder 730 is coupled with transport processor 720 andreceives its output. In one embodiment, FEC encoder 730 uses parallelconcatenated convolution encoding. However, as will be recognized bypersons of skill in the art, other coding techniques are possible.Modulator 750 modulates a baseband signal with the channel bits anddelivers it to a radio/antenna for over-the-air transmission tosatellite 770.

At the receiver 760, the forward link signal is down-converted to abaseband signal. Periodically, receiver 760 is obstructed and losessignal tracking. The receiver employs a signal presence detector (SPD)780 having a blockage-prediction filter to mitigate the effects ofsignal loss on the signal acquisition and tracking loops 790. In someembodiments, SPD 780 detects the presence of the forward link signal andinterruptions of the forward link signal. Blockage prediction filteruses input from SPD 780 to estimate the arrival and duration of ablockage event.

As shown, demodulator 770, signal presence detector 780, and trackingloops 790 are arranged in a loop. In some embodiments, based upon theestimates of its blockage-predication filter, SPD 780 disables errordetector feedback within the receiver's carrier-phase, symbol-phase andautomatic gain control loops. Thus, for example, immediately before apredicted blockage occurs, SPD 780 causes receiver 760 to operate in atemporary open-loop condition. This is called “fly-wheeling”tracking-loops 790 through the outage and it avoids the effect ofinjecting noise-only error detector values into the parameter trackingloops. Otherwise, in some cases, noise-only input can be so detrimentalas to prevent proper reacquisition when the forward link signalreappears. Also, as shown, SPD 780 also generates a return link transmitcontrol signal. As discussed below, the return link transmit controlsignal may be used to enable/disable return link transmissions basedupon blockage conditions.

During temporary open-loop operation, SPD 780 marks bits fromdemodulator 770 as erasures. This is accomplished, in the embodimentshown, by alerting a marking engine 800 that bits received from thedemodulator 770 are erasures. Marking erasures has the beneficial effectof improving the capability of FEC decoder 810 to identify and correctbit errors. FEC decoder 810 outputs a decoded bit stream to transportprocessor 820 which frames the data into packets and ensures the correcttemporal order of the packets provided to the system for processing. Inthis manner, receiver 760 uses information about upcoming blockageevents control tracking loops 790 and thereby minimize link loss.

Focusing again on the return link, it can be desirable to have localcontrol over transmit timing. This is particularly true in the case ofobstructed terminals where, for example, data presented to a modem canbe transmitted without having first to ask for bandwidth allocation andthen to wait for its assignment by a central bandwidth manager. Oneapproach to locally controlling the return link transmit signal is toform an estimate based upon channel blockage characteristics and to usethe estimate to determine when data can be transmitted and when datatransmission should be suspended. The following discussion assumes anestimate is generated at the obstructed mobile platform. In general,however, the estimate could also be made at the gateway.

In various embodiments of the present invention, a signal presencedetector is used to predict the occurrence of a blockage event. At amobile terminal, for example, an SPD circuit (such as SPD 780) can servetwo purposes. First, the SPD can be used to “freeze” tracking loopsduring a blockage event thereby minimizing link loss. As used herein,“freeze” essentially means to hold or stop a tracking loop during ablockage at or near the place the loop was in at the beginning of theblockage. The loop may thus be resumed at or near the end of theblockage, without the disruption of restarting the loops. This allowsthe loop to be placed on hold, or frozen, during the blockage. This wasdiscussed in connection with forward link erasure marking. Second, theSPD can be used to control the timing of return link transmissions bysignaling open transmit times.

To control return link transmission, an estimate of blockagecharacteristics can be formed based upon information from the SPD. Forexample, the SPD can detect when blade rotation starts and stops in ahelicopter-based terminal. Initial values of blockage period andblockage duration parameters can be established based upon signalstrength measurements. When operating conditions change, the SPD candynamically adapt. For example, as a helicopter turns and theblade-obstruction profile changes, the blockage fade depth and durationcan also change significantly. The SPD can be configured to respond tothese changes automatically by adjusting a signal presence/absencethreshold to stay near an optimal level. This, in turn, can feedbackinto the blockage predication determination and can be used to updateand improve the estimate of blockage characteristics.

The blockage estimate can be advantageously used to form “dynamic guardwindows” around predicted blockage times at an obstructed terminal. Thedynamic guard windows provide lead time before a predicted blockageoccurs and lag time after it passes. During the interval defined by theguard windows, return link transmission is suspended. The guard windowsfunction in the presence of changing conditions such as helicopterdynamics or rotor speed to enable tracking algorithms to operatereliably in the presence of such outages. Return link data generatedduring an outage can be stored in a buffer and subsequently transmittedoutside of the guard/suspended interval.

In one embodiment, the return link transmit data stream is suspended onelead time before a predicted blockage and resumes one lag time after theend of the blockage is detected. The lead and lag intervals can bedefined in relation to a data rate of the communication system. Forexample, the lead time may be established to permit transmission of aburst to be completed before the expected arrival of the periodicblockage. Thus, a relatively long lead time may be needed to completeburst transmission at low data-rates and a relatively short lead timemay be needed to complete burst transmission at high data-rates. Lagtime, on the other hand, may be established based upon the time requiredfor tracking loops 790 to reacquire the forward link signal after theblockage has passed.

FIG. 8 is a diagram depicting an antenna blockage event according to oneembodiment of the present invention. As shown, a 1,500 byte IP packet900 is to be transmitted from an obstructed terminal at a data rate of512 kbps. The packet is divided into 53-byte cells for over-the-airtransmission. For purposes of illustration, some of these cells (1, 2, .. . 25) are shown below packet 900.

As packet 900 is being transmitted, an antenna blockage event 910 occursmidway through transmission of cell 8. Two possible outage detectionlead times are shown—a relatively long lead time 920 corresponding tochannel rate of 1 Mbps, and a relatively short lead time 930corresponding to a channel rate of 10 Mbps. The relatively longer leadtime 920 of the 1 Mbps signal provides more time to complete the 1 Mbpstransmission and for the tracking algorithm to reliably track the outagewindow. A lagging guard window is also established at the end ofblockage event 910 as shown by element 950 and defines the point 960 atwhich transmission of packet data on the return link is restarted.

In operation, the SPD may accurately predict that blockage event 910will occur sometime during the transmission of cells 8-10. A lead/lagtime can then be determined for the predicted blockage using theestimated blockage duration and the appropriate channel data rate. Thelead/lag times thus establish dynamic guard windows around the predictedoutage which help to compensate for processing delays that may beincurred during detection of the outage and provide adequate time totransmit one or more pending cells before the blockage arrives. In thefigure, the lead time is show as starting with the arrival of cell 7 ofpacket 900. However, it will be appreciated that lead/lag time valuesare dynamic and can change based upon operating conditions such as ifthe blockage duration or channel bit rate changes.

During the blockage, the return link data steam is suspended. Cellsgenerated during the suspended period may be placed into a buffer andmaintained for the duration of the outage. Thus, as shown, cells 7-25are suspended throughout blockage event 910. Depending upon theimplementation, cells which fall within the applicable guard windows mayor may not be transmitted. Once the end of the blockage is detected(with an appropriate guard period), the transmission of the suspendedcells will resume. Accordingly, transmission of packet 900 resumes withcell 7, following lagging guard window 950 at the tail end of theobstruction.

FIG. 9 describes one potential alternative embodiment. The system ofFIG. 9 includes input data module 902, outer encoder 904, inner encoder906, symbol mapper 908, interleaver 910, correlation data add module911, modulator 912, a blockage impaired data channel 950, demodulator992, correlation data removal module 991, deinterleaver 990, symboldemapper 988, inner decoder 986, outer decoder 984, and output datamodule 982.

Input data module 902 may comprise any computing system which maydeliver data to a communication system as described here. Outer encoder904 and inner encoder 906 may both function as encoders to encode datafor different purposes. In alternative examples, more or fewer encodersmay be used to optimize encoded data to be transmitted across a blockageimpaired channel 950. Symbol mapper 908 may also be referred to as aspreader, and may function to convert bits into chips. Interleaver 910may then function to break up potential long strings of 1 or 0 chips bysorting or randomizing chips in a known fashion. Data correlation module911 may add additional data into a string of chips for the purposes ofdata stream matching as further described below. Modulator 912 mayoutput data to the blockage impaired channel 950. There receiver sidethen performs the inverted action of the corresponding module on thereceiver side. Demodulator 992 may receive the data from the blockageimpaired channel 950, correlation data removal module may remove addeddata and identify blockages as further described below, deinterleaver990 may reverse the randomizing process implemented by interleaver 910,symbol demapper 988 may convert the received chips into appropriatebits, inner and outer decoders 984 and 986 may decode the data to undothe encoding of encoders 904 and 906, and the data may then be output toan appropriate computing system via output data module 982.

Thus, in the above described system, input data stream may flow throughan outer encoder and an inner encoder, the constructed encoded bitstreams may then be mapped to I/Q symbols, which may be further spreadedinto I/Q chips if needed. The I/Q symbols/chips may then be interleaved,and correlation data added, before the signals are frequency modulatedand passed onto a satellite channel where blockage may impact the RFsignal. The receive side may frequency demodulate the signals from thesatellite channel, and deinterleaves the I/Q symbols/chips to recoverthe original symbol/chip stream. If the spreading is used on thetransmit side, then the resultant chip stream may be despreaded into asymbol stream, which is further demapped into a bit stream. The bitstream is decoded via inner and outer decoders to recover any bit errorsencountered, and may thus create the output data stream.

In certain embodiments, interleaver 910 and deinterleaver 990 may modifydata in a transmission channel to disperse potentially large blocks ofcontiguous data corrupted during the blockage into more distributed andsmall chunks of bit errors. In certain embodiments, distributed errorsmay be more readily identifiable and correctable by certain decoders.The interleaver/de-interleaver modules may thus mitigate the effect ofblockages that would otherwise corrupt large blocks of contiguous data.

A convolutional interleaver is one potential embodiment of aninterleaver that may be used on the transmit side and a convolutionaldeinterleaver may be used on the receive side. FIGS. 10 and 11 thereforeshow one potential embodiment of an interleaver and deinterleaver inaccordance with one embodiment. FIG. 10 shows data input 1010, dataoutput 1090, serial to parallel converter 1020, parallel to serialconverter 1080, and delay lines 0 through M−1. While FIG. 1 shows animplementation with M delay lines for systems having M-chips in a blockof data, other embodiments may have any number of delay lines or chipsper block. In FIG. 10, Each M-chip block of serial data input to theinterleaver is output in parallel to the branches 1 through M−1. Eachbranch has a different delay attached to it. In the diagram, J is thedelay unit, and making (M−1)*J the longest delay line/branch. Followingthe branch structure, the 0^(th) chip goes straight through to theparallel to serial block on delay line 0, the 1^(st) incurs a J lengthdelay, and so on, with the (M−1)^(th) branch incurring a J*(M−1) delay.Hence, at a given time, a shuffled set of M chips arrive at the parallelto serial converter 1080. Here, these chips are serialized and output atdata output 1080 as part of the interleaved frame. Both the size of theblock and the delay may be defined by an interleaver register. Thisregister may be programmable by software and may be defined as a part ofsetup for the receiver. Distinct pairs of block size and delay may beused for the corresponding end-to-end delay setting.

The deinterleaver structure of FIG. 11 is only different from theinterleaver structure of FIG. 10 in that the order of the branch delaylines is reversed to guarantee that a fixed delay of (M−1)*J is achievedon all branches after the interleaver and the deinterleaver operations.FIG. 11 shows data input 1110, data output 1190, serial to parallelconverter 1120, parallel to serial converter 1180, and delay lines 10through N−1. The number of delay lines in the interleaver may be set tomatch the number of lines in the deinterleaver, but any number of delaylines may be used as long as the deinterleaver along with any additionalstructure functions to undo the process of the interleaver.

Just as in the system describe above in FIG. 7, the system of FIG. 9 mayadditionally function to improve communication by detecting blockages inchannel 950. Such detection may be based on a power anomaly detectionmethod, a data-aided correlation amplitude thresholding method, or anyother method. The system of FIG. 9 uses a power amplitude thresholdingmethod implemented using data correlation modules 911 and 991. In thedata-aided correlation amplitude thresholding method, known datapatterns, which may be referred to as unique identifiers, may beinserted periodically into the interleaved chip stream on the transmitside by correlation data add module 911, and the receiver correlates theincoming chip stream at the unique identifier positions with the localcopy of the unique identifiers and evaluates the correlation amplitudewhen the unique identifiers are received by correlation data removalmodule 991. In certain embodiments, when the receiver is in trackingstate, the correlation value may be consistently above a programmablethreshold unless the signal is blocked, and thus the correlation valuedips through the threshold marks at the start of a blockage and recoversto over the threshold marks the end of the blockage. The blockage maythus be identified by a low correlation value for the unique identifiersreceived at the correlation data removal module 991 compared with theexpected unique identifier which is known to be incoming.

The correlation over the identifier length window as produced by thephase error detector of the carrier phase loop may also be used to anearly and raw indicator of the signal strength, given that the carrierphase loop is in the tracking mode now and the residual phase error maybe small. As a result, in the tracking mode, the data-aided correlationmay have a large positive value on an I channel and small value on a Qchannel when the signal is present. However, during the blockage whenthe identifier signal is blocked, noises are fed into the phase errordetector, the correlation should result in small values on both I and Qchannel.

Correlation data removal module 991 may create a raw low signal flag ina data tracking mode whenever the larger amplitude on the correlatoroutput is less than a configurable threshold. In certain embodiments,this flag may be sent through a simple filter, and a signal blockageindicated when the low signal flag occurs at least a certain number ofconsecutive times. The signal blockage or absence indicator may then beused to zero data in correlation data removal module 991 and the zerodata passed to de-interleaver 990. The zero data may then bede-interleaved with the received unblocked data, and the errorsintroduced by the zeros corrected by decoders 984 and 982.

In certain embodiments, a demodulator 992 may have an acquisition modethat may put the demodulator 992 on hold for a significant amount oftime, during which all the data is lost. In order to maintain aneffective data link during the blockage, the system may function toavoid a decoder signal acquisition process. During some long blockageshowever, a demodulator 992 may lose a lock and may force an acquisitionmode. In certain embodiments, therefore, large timeout timer may be setin a demodulator tracking state for the blockage applications so that itwill stay in tracking state for at a certain amount of time even if thesignal is unlocked. In other words, when demodulator 992 loses signaltracking, a delay may be implemented to prevent demodulator 992 fromentering an acquisition mode. In certain embodiments, an acquisitionmode may put the decoder on hold for up to 250 ms, and a 50 ms timeouttimer may be sufficient to allow correlation data removal module toidentify a blockage before a demodulator 992 enters an acquisition mode.The correlation data removal module 991 may then freeze the demodulator992 tracking to further prevent entry into an acquisition mode andcorresponding data loss.

To ensure the demodulator 992 stays in the tracking state, correlationdata removal module 991 may thus intervene in the carrier phase loop,the time tracking loop, and the power tracking loop so that they wouldnot drift away due to bad samples during the blockage, and this can beaccomplished by zeroing both the phase error detector output, chip errordetector output, and power detector output when correlation data removalmodule 991 identifies a blockage and outputs a signal blockage or signalabsence indicator.

In certain embodiments, receiver software may set a modcode mask basedon an averaged signal, and a blockage may cause this average tofluctuate significantly, which in turn may cause the modcode mask tochange drastically. This may result in a frame not being detected anddemoded with the right modcode during incorrectly blockage. In certainembodiments, this may be resolved measuring signal strength datasupplied to the receiver software, and a flag may be set whenever asignal absence is indicated by correlation data removal module 991during a signal accumulation window. The receiver software then usesthis flag to exclude any estimate that has used the tagged signalstrength data samples, and thus keeps the averaged signal above the sameaverage with the blockage. With this scheme, the average signal estimatemay not alter and may have a same distribution as without the blockage.

Further, correlation data removal module 991 may intervene in anautomatic gain control (AGC) loop during a blockage. The AGC loop may bedriven to a wrong set point due to the blocked signals, and an AGC loopintervention freezing the AGC based on the signal absence indicator mayfunction to combat this effect.

As described above, the detection of signal absence may be used to avoidsignal loss, demodulator acquisition modes, and loop failure in areceiver. In alternative embodiments, as described above, a poweranomaly detection method may be used. In such a power anomaly detectionmethod, a system may integrate I/Q chip power measurements, and use thepower anomaly corresponding to significant power increases afterblockage and power decreases during blockage to mark the start and theend of a blockage.

The present invention can be implemented in digital electroniccircuitry, or in combinations of computer hardware, firmware, andsoftware. An apparatus according to the invention can be implemented ina computer program product tangibly embodied in a machine-readablestorage device for execution by a programmable processor as a program ofinstructions for performing functions of the invention by operating oninput data and generating output.

The present invention has many applications. In describing an embodimentof a satellite communication system according to the present invention,only a few of the possible variations are described. Other applicationsand variations will be apparent to one of ordinary skill in the art, sothe invention should not be construed as narrowly as the examples, butrather in accordance with the appended claims.

What is claimed is:
 1. A method for receiving data at a terminal via anintermittently obstructed channel in a communication system, the methodcomprising: tracking symbol characteristics of a signal received via theintermittently obstructed channel using one or more tracking loops;demodulating the signal according to the tracked symbol characteristicsto obtain demodulated data samples; decoding the demodulated datasamples using a forward error correction code to obtain a series ofcorrected data packets; identifying a blockage event for theintermittently obstructed channel; and suppressing tracking for at leastone tracking loop of the one or more tracking loops during the blockageevent.
 2. The method of claim 1, further comprising: replacing thedemodulated data samples during the blockage event with zeros.
 3. Themethod of claim 1, wherein the identifying the blockage event comprises:identifying a start and an end of the blockage event based on changes inreceived power of the signal.
 4. The method of claim 1, furthercomprising: identifying an end of the blockage event; and resuming thetracking for the at least one tracking loop at the end of the blockageevent.
 5. The method of claim 1, wherein the identifying the blockageevent comprises: receiving periodic unique identifiers at the terminal,wherein the received periodic unique identifiers are spaced atpredetermined time periods on the signal; correlating the receivedperiodic unique identifiers with a copy of the periodic uniqueidentifiers stored at the terminal; and determining that the correlationfor at least a portion of the periodic unique identifiers is below apredetermined threshold to identify the blockage event.
 6. The method ofclaim 1, further comprising: determining a modcode mask for detection ofa modcode of the signal based on an averaged signal strength, whereinthe averaged signal strength excludes signal strength estimates duringthe blockage event.
 7. The method of claim 1, further comprising:setting a delay timer for entering a signal acquisition mode afterlosing a lock on the tracked symbol characteristics to be greater thanan expected duration of the blockage event.
 8. The method of claim 1,wherein the at least one tracking loop comprises any of a carrier phaseloop, a time tracking loop, a power tracking loop, or combinationsthereof.
 9. The method of claim 1, wherein the suppressing the trackingcomprises zeroing an error output of the at least one tracking loop. 10.The method of claim 1, further comprising: deinterleaving thedemodulated data samples to obtain deinterleaved data samples for inputto the decoding.
 11. The method of claim 10, wherein the deinterleavingthe demodulated data samples comprises deinterleaving the demodulateddata samples using a convolutional deinterleaver.
 12. The method ofclaim 1, wherein the blockage event is due to obstruction of theintermittently obstructed channel at the terminal.
 13. The method ofclaim 1, wherein the terminal comprises a satellite terminalcommunicating with a gateway via a satellite.
 14. A receiver apparatusfor receiving data at a terminal via an intermittently obstructedchannel in a communication system, the receiver apparatus comprising: ademodulator that receives a signal via the intermittently obstructedchannel and tracks symbol characteristics of the signal using one ormore tracking loops, the demodulator demodulating the signal accordingto the tracked symbol characteristics to obtain demodulated datasamples; a decoder that decodes the demodulated data samples using aforward error correction code to obtain a series of corrected datapackets; and a correlation data removal module that identifies ablockage event for the intermittently obstructed channel and suppressestracking for at least one tracking loop of the one or more trackingloops of the demodulator during the blockage event.
 15. The receiverapparatus of claim 14, wherein the correlation data removal modulereplaces the demodulated data samples with zeros during the blockageevent.
 16. The receiver apparatus of claim 14, wherein the correlationdata removal module identifies a start and an end of the blockage eventbased on changes in received power of the signal.
 17. The receiverapparatus of claim 14, wherein the correlation data removal module:identifies an end of the blockage event; and resumes the tracking forthe at least one tracking loop at the end of the blockage event.
 18. Thereceiver apparatus of claim 14, wherein the correlation data removalmodule: receives periodic unique identifiers at the terminal, whereinthe received periodic unique identifiers are spaced at predeterminedtime periods on the signal; correlates the received periodic uniqueidentifiers with a copy of the periodic unique identifiers stored at theterminal; and determines that the correlation is below a predeterminedthreshold for at least a portion of the periodic unique identifiers. 19.The receiver apparatus of claim 14, wherein the receiver apparatusdetermines a modcode mask for detection of a modcode of the signal basedon an averaged signal strength, wherein the averaged signal strengthexcludes signal strength estimates during the blockage event.
 20. Thereceiver apparatus of claim 14, wherein a delay timer for thedemodulator for entering a signal acquisition mode after losing a lockon the tracked symbol characteristics is set to be greater than anexpected duration of the blockage event.
 21. The receiver apparatus ofclaim 14, wherein the at least one tracking loop comprises any of acarrier phase loop, a time tracking loop, a power tracking loop, orcombinations thereof.
 22. The receiver apparatus of claim 14, whereinthe correlation data removal module suppresses tracking for the at leastone tracking loop by zeroing an error output of the at least onetracking loop.
 23. The receiver apparatus of claim 14, furthercomprising: a deinterleaver that deinterleaves the demodulated datasamples to obtain deinterleaved data samples; and a symbol demapper thatdemaps the deinterleaved data samples following the deinterleaving andprior to the decoding.
 24. The receiver apparatus of claim 23, whereinthe deinterleaver comprises a convolutional deinterleaver.
 25. Thereceiver apparatus of claim 14, wherein the blockage event is due toobstruction of the intermittently obstructed channel at the terminal.26. The receiver apparatus of claim 14, wherein the terminal comprises asatellite terminal communicating with a gateway via a satellite.